Method in connection with the production of paper or cardboard

ABSTRACT

Method for manufacturing paper or cardboard in one or more plies in order to achieve a paper or cardboard product with an even quality. The method comprises determining a fiber composition of a broke paper or cardboard waste broke, fractionating the broke; and conveying at least one fiber fraction obtained thereby to a given ply in the paper or cardboard. In this way the fractionating is controlled so that the fiber composition in the fiber fraction obtained corresponds to a fiber composition in the input paper pulp for the given ply.

TECHNICAL AREA

The present invention relates to a method in connection with themanufacturing of paper and cardboard. The method aims to level outquality variations and to raise the quality level through convertingexcess waste for manufacturing, so-called broke. The converted broke isreturned in a more controlled way to the paper of cardboard, as comparedto conventional techniques.

BASIS FOR THE TECHNIQUE

When manufacturing paper and cardboard from cellulose fibres, greatemphasis is placed on the fibre composition for different products inorder to utilise the fibres' properties in the best way. So, forexample, paper and cardboard are manufactured from different stockcompositions consisting of different proportions of different fibre andpulp types depending on which properties are desired in the finalproduct. Even if a given paper or cardboard quality has a given nominalfibre composition, there is a natural variation in the fibre rawmaterials included as different wood species have different fibrecharacteristics, see Table 1, and there are also natural propertyvariations for the fibres in wood with regard to length, width, fibrewall thickness, etc. The fibre length distribution for a Swedishsoftwood pulp spans from fractions of millimeters up to 6-7 mm. Forbirch pulp, the corresponding value is from fractions of millimeters upto approx. 3-4 mm. This means that fresh cellulose pulps demonstratemajor non-homogeneities in fibre property distributions.

TABLE 1 Fibre dimensions for several different timber types Southernstate pine Loblolly pine, U.S.A. Swedish pine spring summer springsummer Euca- wood wood wood wood lyptus Birch Average length, 3.0 3.52.7 3.0 1.0 1.1 mm Average width, 45 35 35 25 16 22 μm Lumen diameter,32 12 30 10 10 16 μm No. of fibres 1 1.5 13 8 per gram timber, x107Source: (Norman B. “Pappersteknik”, 1991, Stockholm, Institute for PaperEngineering, Kungliga Tekniska Hogskölan)

When manufacturing paper and cardboard, there is always produced aquantity of excess material, so-called broke. This broke can compriseedge strips, widths on rolls of final product which results in that thewhole machine width cannot be used to the full, second-rate quality,etc. From the mentioned examples of factors which result in broke, it isunderstood that the broke quantity varies over time. If one manufacturesa product which complies with the quality specification and utilises thewhole machine width, there will be small quantities of broke. When forsome reason there are problems with complying to the qualityspecification, e.g. at a change of quality on the machine, and the fullmachine width cannot be used, the quantities of broke will becomelarger.

Table 2 shows a calculation example of how a varying broke quantitychanges the fibre composition in a three-ply product for different brokemixes. The example is based on the broke being returned directly to theinner ply.

TABLE 2 Share of the total basis weight of fibre composition for topply, inner ply and bottom ply in the finished product, both nominallyand for the three different broke mixes. Broke mix % of total weightNominal 0 10 20 Top ply (%) 25 25 27.5 30 Inner ply (%) 50 50 45 40Bottom ply (%) 25 25 27.5 30

All paper and cardboard qualities contain larger or smaller nominalquantities of broke. Only in cases where the whole quantity of broke canbe used directly, with or without subsequent processing, qualityvariations due to nominal fibre composition in the paper or cardboardproduct is avoided, i.e. if e.g. 15% of the production results in brokewhen a quality which nominally should contain 15% broke of theproduction is manufactured at the same time. In all other cases, thebroke will contribute to a quality variation by a varying fibrecomposition which deviates from the nominal fibre composition, due tothe fact that there is a surplus of broke which must be stored, a lackof broke, stored broke of a different fibre composition compared to thequality which is presently being manufactured, etc. Broke quantitiesvarying over time will sooner or later result in quality variations forall paper and cardboard qualities.

According to known technique, the broke can be managed in different waysdepending on the broke quantity. Conventionally, the disintegrated brokeis recycled, either directly, after storage in a vat/tower or afterstorage as a roll, to the paper or an inner ply for multi-ply paper andmulti-ply cardboard. In the ideal case, the broke is broken up in water,where appropriate subsequently processed through beating or refining,and re-used in the production directly together with the originallyincluded cellulose pulps. The varying broke quantities will howeveroften result in broke having to be stored. This storage can be performedin two ways. One way is that the disintegrated broke is stored in astorage vat/tower after the disintegration and possible subsequentprocessing. The other way is to have rolls of a outclassed quality instock, which is disintegrated and where appropriate subsequentlyprocessed for use as needed.

In the case of multi-ply paper or cardboard, it is not only varyingbroke quantities that cause problems. By returning the broke to an innerply in the cardboard, the inner ply to which the disintegrated broke isadded will be brought to contain pulp with fibres that originate from anouter ply. The multi-ply paper or cardboard will thereby, calculated onits total weight, contain a larger proportion than nominally of fibresof the type which is intended to be used in an outer ply of the paper orcardboard and a smaller proportion than nominally of fibres of the typeintended to be used in an inner ply of the paper of cardboard, seeexamples in Table 2.

It is known that the fibres in a fibre flow can be fractionated by meansof a screen or hydrocyclone, a screen being used to fractionate fibresprimarily according to fibre length, while a hydrocyclone is used tofractionate fibres with different thicknesses and thereby differentfibre flexibilities. Studies have shown that with the aid of sizefractionating (screen), it is possible to separate out a large share ofshort fibres from a fibre flow; Fredlund M. et al., “Förbättradekvalitetsegenskaper hos kartong genom fraktionering”, STFI-rapport TF23, 1996, Stockholm, STFI; Grundström K-J, “STFIs silteknik höjerkvaliteten vid kommersiell drift”, STFI Industrikontakt, 1995, no. 1, p.7-8. It has also been documented that by using a hydrocyclone one canseparate flexible fibres from more stiff fibres; Wood J. R. and KarnisA., “Distribution of fibre specific surface of papermaking pulps”,Pulp&Paper Canada 80 (1979):4, p. 73-78, Bliss T., “Secondary fibrefractionation using centrifugal cleaners”, Tappi Pulping Conference,1984, 217 pp; Paavilainen L., “The possibility of fractionating softwoodsulphate pulp according to cell wall thickness”, Appita 45 (1992):5, p.319-326. In U.S. Pat No. 5,002,633 there is described a fractionatingprocess which aims to separate the longest fibres from short fibres,fillers, contaminants, etc., from a pulp for re-use of the longestfibres in paper manufacturing.

Moreover, it is known to combine different fractionation equipment infractionation systems for different purposes. In U.S. Pat No. 5,403,445,recycled fibres for manufacturing of paper with more than 70% recycledfibres are fractionated, and in U.S. Pat No. 5,061,345 a series ofscreens is used to separate out fibres from filler. In somefractionating systems, the aim is to separate fibres with differentproperties in order to be able to use the fibre fractions in differentplies. This is described in U.S. Pat No. 5,147,505 where the fibres in apulp are separated according to coarseness and the rougher fibres areused in one ply and the more slender fibres are used in another ply. InEP 0653516 A1, it is mentioned in a similar way that softwood fibres areseparated into a fraction with thick-walled fibres which are used in oneply and a fraction with thin-walled fibres which are used in anotherply.

In Vollmer H., “Simulering av fraktioneringssystem”, STFI Report TF 81,1997, STFI, Stockholm, it is described how fractionators can becharacterised for different operating conditions and how they distributefibres with different properties in different fractions for givenoperating conditions. Thereby it is possible to predict the fibreproperty composition in the resulting fibre fractions when the operatingconditions for combinations of given characterised fractionators areknown, and when the fibre property composition of the input fibre flowis known.

It is also known per se to determine fibre characteristics on-line. Suchsystems are described e.g. in Fransson P-I., “Mätningar med STFIFiberMaster i ett kartongbruk”, STFI Report TF 74, 1997, STFI,Stockholm; Karlsson H. et al., “STFI FiberMaster”, STFI Report TF 70,1997, STFL, Stockholm; Thomsson L. et al., “Uppskattning av andelen CTMPi centerskikt vid kartongtillverkning”, STFI Report TF 78, 1997,Stockholm, STFI.

As a result of natural variations in the fibre raw material, varyingbroke quantity and varying broke composition, it is realised that it isimpossible to completely avoid quality variations. Nevertheless, thevariations constitute a problem in connection with increasingrequirements on paper and cardboard qualities. At increasingrequirements on efficient production and lower production costs, theneed for optimum use of the fibre material is accentuated. This entailsa need for all fibre material used for paper and cardboard manufacturinghaving to be used in the best way, i.e. the fibres should be used forthat which they are best suited for. None of the above-mentioneddocuments discuss the problem of being able to manage broke formanufacturing of paper or cardboard and to be able to implementfractionation of this broke, whereupon the fractionation can becontrolled so that different types of fibre in the broke can be conveyedto the most suitable ply/plies, in controlled proportions, in the paperor cardboard as produced.

DESCRIPTION OF THE INVENTION

By the present invention, a method in connection with the manufacturingof paper of cardboard is presented, whereupon excess material from themanufacturing, so-called broke, is re-used in an optimum way. Throughthe invention, the quality of the product may be controlled, theproduct's service properties may be improved and quality variations inthe paper or cardboard product may be levelled out.

These and other objects are achieved by means of the method according tothe invention as described below.

According to one aspect of the invention, the fibre composition for thebroke is determined by on-line characterisation of one or more of thefibre length, fibre width, fibre coarseness, fibre shape and fibreflexibility parameters, while the fibre composition in the fibrefraction(s) produced is determined in the same way or is calculated, andthe fibre composition in the input paper pulp for the said given ply isdetermined by means of intermittent characterisation of one or more ofthe said parameters. Moreover, the fractionation is controlled on basisof one or more of the parameters fibre composition in the input paperpulp for the said given ply, fibre composition in the broke and fibrecomposition in at least one of the fibre fraction(s) produced.

According to another aspect of the invention the fractionation equipmentused for the fractionation is characterised intermittently in terms offractionating effect for different fibre compositions furnished to thefractionation equipment and operating conditions, whereby operatingconditions refer to the input flow to the equipment, the ratio betweeninput flow and reject, the concentration of the input flow, or similaroperating conditions, which characterisation forms the basis for thesaid control of the fractionation. The fractionation is controlled,preferably continuously, by means of a change of at least one of theoperating conditions of the fractionation equipment, which operatingconditions include the input flow to the equipment, the ratio betweeninput flow and reject, the concentration of the input flow, or similaroperating conditions.

According to another aspect of the invention, the fractionation isperformed in at least two steps, whereupon a first fraction iscontrolled primarily to contain short fibres, and another fraction iscontrolled primarily to contain long fibres. The fraction with longfibres is controlled through fractionation in a second step to consistof a second fraction primarily containing long flexible fibres and athird fraction primarily containing long stiff fibres, after which thesaid first and/or second and/or third fraction is distributed in adesired proportion to the said given ply or several given plies at themanufacturing the paper or the cardboard.

According to a further aspect of the invention, the fractioning isperformed on basis of fibre length, preferably by use of a screen, whilethe fractionation on basis of fibre thickness and thereby fibreflexibility is performed preferably by use of a hydrocyclone. Using themethod according to the invention, the broke in existing chemical shortfibre pulp (preferably in the above-mentioned first fraction), chemicallong fibre pulp (preferably in the above-mentioned second fraction whichcontains long flexible fibres) and mechanical pulp (preferably in theabove-mentioned third fraction which contains long stiff fibres) may bereturned in a desired proportion to the desired ply, which gives ahigher and more even quality in the product, since despite varying brokequantity and broke composition it is possible to control the product toa nominal fibre composition in the ply/plies.

In the case of multi-ply paper or cardboard, using the invention, in thebroke existing

chemical short fibres may be returned preferably to their original outerply for which strict requirements for surface properties are imposed,

mechanical pulp may be returned preferably to its original inner ply forwhich requirements are imposed for filling,

chemical long fibres may be used optionally, after possible subsequentbeating and/or fractionating, in an outer ply and/or as reinforcement inan inner ply. If the fraction containing mainly chemical long fibresundergoes further fractionation, the fine fraction can be conveyed to anouter ply and the coarse fraction can be conveyed to an inner ply asreinforcement, after beating.

The advantage of the method according to the invention is that byseparating the different fibre components in the broke, a desiredproportion of the fibre component can be controlled to be included in acertain ply, in a specific proportion, in the final product. Inparticular, on-line fibre characterisation, characterisation offractionators and calculation of the fibre property composition ofdifferent fractions makes it possible to utilise fractionation in orderto, with the aid of suitable combinations of fractionation equipment,achieve a very good possibility for optimum control of the fractionationat every individual step. Several subsequent fractionation steps maytogether create a fractionation system in order to tailor-make fractionswith the desired fibre property composition. The desired fibre propertycomposition in a certain fraction may thereafter be controlled in adesired proportion, on a par with the nominal fibre propertycomposition, to be included in a desired ply. The product, i.e. thepaper or cardboard, will thereby achieve a good quality and evenness inthis good quality, despite the broke being included in the process.

BRIEF FIGURE DESCRIPTION

In the following the invention will be described with reference to thefigures, of which:

FIG. 1 shows a simplified diagram of a proposed embodiment of theinvention,

FIG. 2A shows a graph that constitutes an example of fibre accept degreeas a function of fibre length for a screen,

FIG. 2B shows a graph that constitutes an example of fibre accept degreeas a function of fibre length for a cyclone,

FIG. 3 shows how the line in FIG. 2A changes in connection with anincreasing ratio between reject and inject in a screen,

FIG. 4 shows a result of a comparison between a bending stiffness indexfor a cardboard according to the invention and a reference cardboard.

DETAILED FIGURE DESCRIPTION

A proposed embodiment for the invention is shown in simplified form inFIG. 1. Based on the figure, one aspect of the invention can bedescribed in a number of steps.

Step 1. Characterisation of Pulp

Each pulp 1, 2, 3 which is intended for a respective ply in the producedcardboard or paper is analysed with on-line fibre characterisationequipment 4. Thereby, the pulp is analysed in terms of different fibreproperties/fibre compositions, e.g. fibre length, fibre width, fibrecoarseness (fibre length weight), fibre form, fibre flexibility, etc. Asufficient quantity of fibres is analysed to achieve distributions withregard to the different properties for every pulp. These fibreproperties/fibre compositions are used as a reference in the subsequentcharacterisation of the fractionator.

When the characterisation of the pulp/pulps 1, 2, 3 is done, the fibreproperty distribution(s) of the pulp/pulps is/are clarified.

Step 2. Characterisation of Fractionator

The fractionator or fractionators 5, 6 which is to be characterised aresupplied with inject pulp which has been characterised in terms of fibreproperties according to step 1. The operating conditions and equipmentfor the fractionator are varied systematically during thecharacterisation experiment. Equipment refers, for example, to the typeof screen basket used if the fractionator is a screen, or the type ofoutlet nozzle used if the fractionator is a cyclone 6, i.e. theequipment determines design-related limitations for the fractionator.Operating conditions refer, for example, to inject flow, ratio betweeninject flow and reject flow, the concentration of the inject pulp, i.e.parameters which determine how a certain fractionator with a certaindesign is operated.

During systematic variation of the operating conditions and theequipment, the reject and accept flows are characterised in the same wayas the inject flow according to step 1. If the equipment is not intendedto be modified, it is sufficient to vary the operating conditionssystematically. As the fibre property distributions for inject, rejectand accept are determined, accept or reject curves are calculated forevery operating condition of the fractionator. A diagrammatic example isshown in FIGS. 2A and 2B.

The graphs in FIGS. 2A and 2B are interpreted such that of all fibres ina given population of a certain fibre length a certain share ends up inthe accept and the remaining share in the reject. For example, of fibreswith length x₁, y₁% end up in the accept. Consequently, 100-y₁% ends upin the reject. Of fibres with length x₂, y₂% end up in the accept and100-y₂% ends up in the reject. The two graphs in the figure alsoillustrate the fact that has been described previously, that the screen5 fractionates according to fibre length, while the cyclone 6fractionates according to other fibre properties (the same fibre acceptdegree irrespective of fibre length). At the change of an operatingparameter for a screen or cyclone, the curve in the respective graph ismoved as is shown in FIGS. 2A and 2B. FIG. 3 shows in a diagrammaticform what happens if the flow ratio between reject flow and inject flowin a screen is varied. An increasing flow ratio thereby leads to areduced proportion of fibres of a certain length ending up in theaccept.

When the characterisation of a given fractionator is finished, it hasthus been clarified how the fractionator shall be designed and operatedin order to, with a given inject with characteristic fibre propertydistributions, achieve a desired accept and reject in terms of fibreproperty distributions.

Steps 1 and 2 described above do not need to be performed continuouslyin the application of the invention, especially not the characterisationof the fractionator. On the other hand, it may be valuable to haveavailable a continuous updating of the characteristics of the inputpulps. The characterisation of the fractionator is relevant as long asthe fractionator is intact, but needs to be repeated if the equipment ischanged or if the operating parameters are changed so that they deviatefrom the intervals within which the different parameters were variedduring the characterisation. The characterisation of input pulp may, ifit is not performed continuously, be redone when the pulp productionprocess is modified, the wood collection area is changed, major seasonalvariations exist, etc.

Step 3. Characterisation of Broke

The part of the manufactured paper of cardboard which is returned asbroke 7 is characterised in the same way as the input pulps in Step 1.The broke is hereupon disintegrated and, where appropriate, processedwith beating or refining.

When the characterisation of the broke has been done, a basis isachieved using the fibre property distributions obtained for the brokefor the way in which the fractionator should be controlled in order toachieve the desired accept and reject in terms of the fibre propertydistributions.

Step 4. Control of Fractionator

With the knowledge from Step 2 of how every fractionator 5, 6 works fora given fibre property distribution, the fibre property distributions ofthe broke 7 from Step 3 form the basis for controlling the operatingconditions of the fractionator 5, 6 so that the fibre propertydistributions for the fractions of the broke, at a comparison with thefibre property distributions for the input pulps 1, 2, 3 are as similaras possible. As the fibre property distributions for the original inputpulps 1, 2, 3 are known from Step 1, based on the fibre propertydistribution for the broke 7 one can calculate the fibre composition inthe broke. This calculated fibre composition governs the operatingparameters of the fractionator so that the operating condition will givethe desired fibre separation. The fractionation is thereby controllede.g. so that there is produced a first fraction 8 chiefly consisting ofa first type of fibres, which resemble a first 1 of the input pulps, asecond fraction 9 chiefly consisting of a second type of fibres, whichresemble a second 2 of the input pulps, and a third fraction 10consisting of a third type of fibres, which resemble a third 3 of theinput pulps, whereupon the different fractions are controlled to arespective ply. By resemblance it is here referred to resemblance infibre composition.

In order to verify that the control is performed in an accurate way,these fractions 8, 9, 10 are characterised 4 in the same way as theinput pulps 1, 2, 3 in Step 1. The comparison between the fibre propertydistributions of the input pulps and the fibre property distributions ofthe fractions shows whether a possible adjustment of the operatingconditions is necessary, which is then performed automatically. Thephysical control of the fractionators 5, 6 is performed by gatheringdata concerning fibre characterisation in a process computer where allnecessary data processing is performed. Depending on the outcome of thedata processing, the process computer thereafter gives signals to theprocess equipment, e.g. adjustments of valves, pumps, etc., in order tocontrol the operating conditions or issue alarms if the equipment forthe fractionators 5, 6 should be modified.

Steps 3 and 4 should be performed continuously during operating in orderfor the invention to operate in the best way.

The four steps described constitute a system for separating the broke 7by, using knowledge of the fibre property distributions of the inputpulps 1, 2, 3, (step 1) and the work method used by the fractionators 5,6 (step 2), analysing the broke (step 3) and controlling thefractionator (step 4) so that fibre fractions are obtained from thebroke which in their characteristics resemble the input pulps.

When the broke is divided into a number of fractions, whose fibreproperty distributions largely conform with the original pulps, thepossibility is given for raising the quality level and the possibilityfor obtaining a more even quality by being able to return the desired,controlled quantities of the different fractions to the paper orcardboard. If it is a multi-ply product, a possibility is also given forreturning a desired and controlled quantity of a certain fraction to acertain ply. If one wishes, in this way, to have a constant recycledquantity of a certain fraction to a certain ply, depending in thenominal fibre composition in the paper or cardboard, it may be requiredthat the different fractions are subject to intermediate storage in astorage vat or a storage tower. In order to make optimum use of thefractions, it may also be advantageous subsequently to process thefractions with beating or refining. This can be done on-line if thefractions are not subject to intermediate storage, or in connection withthe intermediate storage of the fractions.

Intermediate storage is especially advantageous if the paper orcardboard produced only consists of one ply. According to the invention,there is then a possibility to convey a fraction of broke from apreviously manufactured paper or cardboard to this ply, which fractionexhibits a fibre composition which resembles the fibre composition inthe single-ply product. The same principle is naturally also applied inconnection with intermediate storage at the manufacturing of multi-plyproducts. Intermediate storage gives an extra possibility for achievingstability in the products. The invention is especially preferred for usein connection with the manufacturing of paper or cardboard with two ormore plies.

EXAMPLE

The following example is based on results from a pilot-scale trial.During the trial, a three-ply cardboard was manufactured with a nominalbasis weight of 200 g/m² on a pilot paper machine. The outer pliesnominally had basis weights of 40 g/m² each, and the central plynominally had a basis weight of 120 g/m². In the reference cardboard theouter plies consisted of a 50/50 mix of chemical short fibre/chemicallong fibre and the central ply consisted of a 50/50 mix of mechanicalfibres/broke. The reference cardboard was compared with a trialcardboard which had the same nominal ply basis weights and the sameoriginal pulps. The difference was that the broke had been fractionatedin three steps. In the first fractionation step, which was performed ina screen, a fraction was separated out which was denoted chemical shortfibre_(broke). In the second fractionation step, the long fibres wereseparated out in a fraction of flexible fibres, chemical longfibre_(broke) and a fraction of stiff fibres, mechanical fibre_(broke).The chemical long fibre_(broke) fraction was fractionated in a thirdstep to an accept consisting of shorter, more slender fibres, chemicallong fibre_(broke, unbeaten) and a reject consisting of longer coarserfibres, chemical long fibre_(broke, beaten). The reject was beaten hardto serve as a reinforcement pulp. The composition in the outer plies ofthe trial cardboard was 50/50 (chemical short fibre+chemical shortfibre_(broke))/(chemical long fibre+chemical longfibre_(broke, unbeaten)) and the composition of the centre ply was 55/45mechanical fibres/(mechanical fibres_(broke)+chemical longfibre_(broke, beaten)). By returning the chemical fibres which in thereference cardboard were found in the centre ply to the outer plies inthe trial cardboard, a significant bending stiffness increase wasachieved as the chemical fibres removed from the centre ply could bereplaced with an increased quantity of mechanical fibres. FIG. 4 showsthe improved bending stiffness expressed as a bending stiffness index.The pilot-scale trial could be verified with laboratory experimentswhere three-ply laboratory sheets were manufactured from the same stocksas were used in the pilot trial. The trial cardboard had approx. a 25%higher bending stiffness index as compared to the reference cardboard.This means that one can manufacture a cardboard with the same bendingstiffness index with 8% lower basis weight. Such a basis weight savingentails decreased raw material costs and thereby a decreased productioncost.

The invention is not limited to the embodiments shown above but can bevaried within the scope of the following claims. It is especiallyrealised that the separation into fractions and the conveying of these,including any intermediate storage, may be varied in intangible ways,whereby each mill obtains a unique possibility to tailor-make itsmanufacturing process. When mills lie within relatively close distancesfrom each other, it may also be profitable to transport fractionsbetween the mills.

What is claimed is:
 1. A method for manufacturing of paper or cardboardin one or more plies in order to achieve a paper or cardboard productwith an even quality, comprising: determining a fiber composition of abroke paper or cardboard waste broke; fractionating said broke; andconveying at least one fiber fraction obtained thereby to a given ply inthe paper or cardboard, whereby said fractionating is controlled so thatthe fiber composition in the fiber fraction obtained corresponds to afiber composition in the input paper pulp for said given ply.
 2. Amethod according to claim 1, wherein said fiber composition for saidbroke is determined by on-line characterisation of one or more of aparameter of fiber length, fiber width, fiber coarseness, fiber form andfiber flexibility, while the fiber composition in fiber fraction(s)obtained is determined in the same way or is calculated, and the fibercomposition in the input paper pulp for said given ply is determined bymeans of intermittent characterisation of one or more of saidparameters.
 3. A method according to claim 1, wherein said fractionationis controlled on the basis of one or more of the parameters in the groupthat consists of fiber composition in the input paper pulp for saidgiven ply, fiber composition in the broke and fiber composition in atleast one of the fiber fraction(s) obtained.
 4. A method according toclaim 1, wherein a fractionation equipment used for the fractionation ischaracterized intermittently in terms of a fractionating efficiency fordifferent input fiber compositions to the fractionation equipment andoperating conditions, whereby operating conditions refer to input flowto the fractionation equipment, ratio between input flow and rejectflow, concentration of input flow, which characterisation forms thebasis for said control of the fractionation.
 5. A method according toclaim 1, wherein said fractionation is controlled by means of a changeof at least one operating condition of the fractionation equipment,which operating conditions include input flow to the fractionationequipment, ratio between input flow and reject flow, concentration ofinput flow or similar operating conditions.
 6. A method according toclaim 5, wherein said fractionation is controlled continuously.
 7. Amethod according to claim 1, wherein said broke, before fractionation,is disintegrated to a fiber suspension which is subsequently processed,where appropriate, by means of beating or refining.
 8. A methodaccording to claim 1, wherein the fractionation of the broke isperformed in at least one step for the production of a first fiberfraction mainly consisting of a first type of fibers, which in fibercomposition correspond to the input paper pulp for a first given ply,and a second fiber fraction mainly consisting of a second type offibers, which in fiber composition correspond to the input paper pulpfor a second given ply, whereafter said first fiber fraction is conveyedto said first ply and said second fiber fraction is conveyed to saidsecond ply.
 9. A method according to claim 8, wherein said first fiberfraction comprises shorter fibers and said second fiber fractioncomprises longer fibers.
 10. A method according to claim 8, wherein saidfirst fiber fraction comprises flexible fibers and said second fiberfraction comprises stiff fibers.
 11. A method according to claims 1,wherein said fractionation is performed in at least two steps, whereby afirst fiber fraction comprises shorter fibers, a second fiber fractioncomprises longer flexible fibers and a third fiber fraction compriseslong stiff fibers, whereafter said first and/or second and/or thirdfraction is distributed in a desired proportion to said given ply orseveral given plies at the manufacturing of the paper or the cardboard.12. A method according to claim 11, wherein the distribution of adesired proportion is performed by comparison of a calculated fibercomposition in a given ply after mixing in fiber fraction(s) with anominal fiber composition in said given ply.
 13. A method according toclaim 11, wherein said second fiber fraction is further beaten and/orfractionated, before being distributed in a desired proportion to one ormore of said given plies.
 14. A method according to claim 1, furthercomprising intermediate storing and, where appropriate, subsequentprocessing by means of beating or refining of one or more of the fiberfractions.